The present invention relates to a process for the control of the quality and of the production of continuous transmission belts and to an automatic installation for the manufacture of such belts.
As is known, transmission belts are elements used to transmit a torque between two devices, such as on earth-moving machines, vehicles, etc.
The belts are of different types and construction, and the transmission can occur by friction in asynchronous belts, or by the engagement of teeth (gearing) in toothed belts. The latter have a plurality of teeth and a plurality of hollow spaces for the engagement with associated toothed pulleys.
The invention will be illustrated with particular reference to the manufacture of toothed belts, but it is not to be considered limited to the use for the production only of this type of belts.
A toothed belt comprises longitudinal inextensible cords buried in a mass of elastomer which has teeth and hollow spaces on one face and is smooth on the other face, known as the back of the belt.
The cords, say of glass fibers, textile or metal threads, etc., are arranged along the neutral axis of the belt and bear the working load, ensuring a constant distance between the axes since the teeth are made of deformable material.
The current manufacturing processes provides for the use of a very long toothed metal drum on which there is initially mounted a sheath of tubular material, corresponding to the drum's internal diameter, used to enhance the resistance of the tooth.
On the drum there is then wound a helical winding of an inextensible thread which will constitute the cords, by means of a machine called a coiling machine, after which successive layers or sheets of uncured rubber or more in general of non-cured elastomeric material.
The semi-finished articles thus obtained, defined as sleeves, are subjected to curing and molding in an autoclave, inside a rubber sleeve or chamber which transmits a molding pressure, while curing heat is provided by steam or water introduced inside the drum.
The rubber or the elastomer filters through the threads of the cords and fills the hollow spaces of the teeth, while the material is pushed inside the teeth on the drum by the pressure of the rubber as it enters.
At the end of the operation, the cylindrical product is cut in a transverse or radial direction in short lengths to obtain the individual belts.
The belts may have defects of different kinds which, also due to the lot system of production, would affect a high number of pieces of finished products. Moreover, the drawbacks due to such defects can appear after several months, say, when the belt is used on a motor vehicle.
It is thus extremely important that quality control of the belts, as well as verification of their actual characteristics, be made during their production, especially in view of the automation of the production processes.
The defects may be originated by many factors. For example, pressure conditions can influence the correct shape of the tooth's profile.
Other defects may derive from the temperature and/or the length of the curing cycle, for example, a crack in the surface can occur after too long a curing cycle.
Other defects may also derive from coiling. In fact, the length of the belt depends on the cords'winding tension, and a non-constant coiling tension can cause the production of belts having a length (distance between the axes) other than that desired.
Some of these defects can also be detected visually (more or less easily) on the sleeve, say, the formation of cracks or bubbles when the belts are at the extremities of the sleeve, while others (such as an out-of-spec distance between the axes due to a non-constant coiling tension) can be detected only after the individual belts have been cut and with suitable measuring and testing machinery.
Moreover, some defects due to production conditions can affect an entire lot of belts, such as, say, those due to incorrect curing conditions, while other defects affect only a certain number of belts in a limited area (for example, a temporary winding irregularity of the coiling machine); while still others can relate to an individual belt (for example, the junction of the cords following a breakage).
According to the known arts, product control was made visually, on a so-called "specula", with which it is possible to examine the sleeve on an illuminated area in order to visually identify any curing defects, say, air bubbles, irregular thicknesses, cracks, etc.
A further final check was also made, preferably by sampling, on the final product, that is, on the individual belts cut from the sleeve.
These production and checking techniques are afflicted by considerable drawbacks and limitations.
These techniques do not in the first place allow any action to be taken in real time when an anomaly is detected on the sleeve or, even worse, on the belt when it has already been cut.
At this point, in many cases, the anomaly is related to a substantial number of belts produced. For example, a defect thus observed in the coiling tension or pitch, shall be corrected only at a later stage and with much delay, that is, after many other belts with the same defect have been produced.
Similarly, an anomalous operation of the autoclave, such as, say, the presence of bubbles on the product due to the formation of condensate during curing, would probably affect the entire lot and, on occasion, even the subsequent lot which has been loaded into the defective autoclave.
In the second place, these prior techniques have the further drawback due to their uncertainty and to the difficulty in detecting all the defects, so that it results in a compromise, often unsatisfactory, between quality of the product and additional production costs due to rejects.
Lastly, with these methods positive indications for a corrective intervention are completely missing: even when the tests are made by suitable checking machinery, the lighting of an alarm pilot light or the signalling of a defect do not, on the whole, provide the operator with indications as to what action to take in the production process.
On the contrary, such an alarm can be an inducement to stop production without there being a real need to take such drastic and costly action in productivity terms.